CN105957886A - Silicon carbide bipolar junction transistor - Google Patents

Abstract

A silicon carbide bipolar junction transistor is formed with a Schottky contact structure on the surface of the extrinsic base region between the edge of the mesa of the emitter (1) and the ohmic contact of the base (2) to form a Schottky barrier on the surface of the extrinsic base region. The Schottky contact structure comprises a base region and a metal layer positioned on the base region. The silicon carbide bipolar junction transistor can prevent electrons from moving to the surface through the Schottky contact structure, inhibit surface recombination and improve the current gain of the device.

Description

A silicon carbide bipolar junction transistor

technical field

The invention relates to the field of high-power semiconductor devices, in particular to a silicon carbide (SiC) bipolar junction transistor (BJT).

Background technique

The wide bandgap semiconductor material SiC is an ideal material for preparing high-voltage power electronic devices. Silicon carbide (SiC) bipolar junction transistor (BJT) is one of the important normally-off devices. Advantages. Compared with Si-based transistors, SiC BJT has the advantages of lower turn-on voltage and no secondary breakdown phenomenon; SiC BJT avoids the gate drive problem of normally-on device SiC JFET, and does not have the disadvantage of large conduction loss of SiC IGBT , there is no problem that the working conditions of the SiC MOSFET are limited due to the poor stability of the gate dielectric and the low mobility of the channel.

The existence of SiC/SiO ₂ high interface state will lead to the instability of the gate dielectric of SiC MOSFET, low channel mobility and other adverse effects; for SiC BJT, the outer base region between the emitter mesa edge and the base ohmic contact of SiC BJT The high interface state density on the surface will become a recombination center, resulting in a large number of minority carriers (electrons and holes) in the base region recombining at the interface to generate a recombination current, reducing the current gain of the device and degrading the device performance. US Patent No. US8378390 proposes a new structure of silicon carbide bipolar junction transistors to reduce the recombination current caused by the SiC/SiO ₂ high interface state. The outer base region between the ohmic contacts uses the metal on the SiO ₂ dielectric layer, the SiO ₂ dielectric layer and the outer base region to form a MOS structure, and uses the BE junction bias voltage to control the potential of the substrate surface of the MOS structure, changing the substrate The carrier density on the surface can suppress the surface recombination current. Although this structure reduces the recombination current and improves the current gain, it still does not fundamentally solve the problem of SiC/SiO ₂ high interface state; and the metal on the SiO ₂ dielectric layer is an electrode that needs to be applied voltage, resulting in this The device of the structure is a four-terminal device, and it is well known in the art that for a triode, a four-terminal device has many disadvantages compared with a three-terminal device.

Contents of the invention

In view of this, the main purpose of the present invention is to provide a silicon carbide bipolar junction transistor to solve at least one of the above technical problems.

In order to achieve the above object, the present invention provides a silicon carbide bipolar junction transistor, a Schottky contact is formed on the surface of the outer base region between the edge of the emitter mesa and the ohmic contact of the base of the bipolar junction transistor. structure.

Preferably, the Schottky contact structure includes a base region and a metal layer on the base region.

Preferably, the Schottky barrier formed by the Schottky contact structure is a P-type anti-barrier layer.

Preferably, the base region is a two-layer structure with different doping concentrations.

Preferably, the two-layer structure of the base region is: the layer near the emitter junction is a base region with a thickness of _t1 , which adopts P-type doping; and the other layer near the drift region is a base region with a thickness of _t2 . region, its doping concentration is greater than that of the base region of the layer close to the emitter junction, reaching the order of 10 ¹⁷ cm ^-3 .

Preferably, the metal layer is not connected to voltage or grounded.

Preferably, the work function of the metal layer is greater than the work function of the surface of the base region.

Preferably, the metal layer is made of nickel or platinum.

Preferably, a SiO ₂ passivation layer is formed between the left end of the metal layer and the edge of the emitter mesa.

Preferably, the thickness of the SiO ₂ passivation layer is 40-60nm.

Based on the above technical scheme, it can be seen that the silicon carbide bipolar junction transistor and its manufacturing method of the present invention have the following beneficial effects: when the work function W _m of the metal is greater than the work function W _s of the P-type semiconductor at the surface of the outer base region, the semiconductor The energy band on the surface bends upward to form a P-type anti-blocking layer (to prevent electrons from moving to the interface), and at the same time, in order to ensure that the emitter junction is forward-biased, the base must be forward-biased, which will make the outer base region away from the energy of the surface The band moves down, the energy band at the surface is further bent upward, and the barrier height increases; the final result is that the surface of the P-type extrinsic base region is in an accumulation state, the hole concentration increases, the electron concentration decreases, and the electrons and holes are separated in space. hole. Due to the existence of the potential barrier, the movement of electrons to the surface is limited, thereby preventing the electrons injected from the emitter region into the base region from being captured by the surface traps in the outer base region, reducing the surface recombination current and improving the current gain; the present invention does not require additional electrodes for the bias voltage, still maintaining the device as a three-terminal device.

Description of drawings

Fig. 1 is the structural representation of a kind of silicon carbide bipolar junction transistor of the present invention;

Fig. 2 is the structural representation of the Schottky contact structure of metal layer and base region of the present invention;

Figure 3 is the energy band diagram before the Schottky contact;

Fig. 4 is the energy band diagram after the contact between the metal and the P-type semiconductor.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

The invention discloses a silicon carbide bipolar junction transistor with a Schottky contact structure, which does not require an additional bias voltage electrode, and still maintains the device as a three-terminal device; and effectively solves the problem of SiC/SiO ₂ The problem of high interface states improves the current gain of SiC BJT devices. More specifically, the present invention makes a Schottky contact structure on the surface of the outer base region between the edge of the emitter mesa and the base ohmic contact of the triode, and forms a Schottky barrier on the surface of the outer base region to prevent electrons Moving toward the surface, suppressing surface recombination and increasing the current gain of the device.

Further, the Schottky structure is formed by the contact between the metal layer and the P-type silicon carbide.

Further, the metal layer should choose a metal with relatively large work function, such as nickel, platinum, etc.; and the metal layer should be floating, not connected to any external voltage, not grounded, that is, not used as an electrode.

Further, the P-type silicon carbide is the base region of the triode, and the base region adopts a two-layer structure with different doping concentrations, and a base region with a thickness of _t1 near the emitter junction adopts low-concentration P type doping, for example, the concentration is less than 10 ¹⁵ cm ^-3 ; another layer close to the drift region is the base region with a thickness of t ₂ , and its doping concentration is higher than that of the base region of the previous layer, reaching 10 ¹⁷ cm ^{- 3} orders of magnitude.

Further, a SiO ₂ passivation layer is formed between the left end of the metal layer and the edge of the emitter mesa, and the thickness of the SiO ₂ passivation layer is 40-60 nm, preferably 50 nm.

The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

In order to increase the current gain of SiC BJT in the prior art, it is necessary to reduce the recombination current at the outer base surface between the edge of the emitter mesa and the ohmic contact of the base. There are three main factors affecting the magnitude of the recombination current:

1. Defect concentration at the surface of the exogenous base region;

2. Electron concentration at the surface of the exogenous base region;

3. Hole concentration at the surface of the extrinsic base region;

Factor 1 depends on the existing material growth and technology level, and factors 2 and 3 may be affected by the design. The present invention is designed to reduce the recombination current on the surface of the outer base region. In this triode, the recombination rate of electron-hole pairs mainly depends on the concentration of minority carriers, and the recombination mainly occurs on the surface of the outer base region between the edge of the emitter mesa and the ohmic contact of the base, and because the P-type base The electrons in the outer base region are minority carriers, so the concentration of electrons on the surface of the outer base region will greatly affect the occurrence of surface recombination. In the present invention, a Schottky contact structure is manufactured on the surface of the outer base region between the edge of the emitter mesa and the ohmic contact of the base of the triode, and a Schottky barrier is formed on the surface of the outer base region to prevent electrons from going to the surface. Movement, reducing the electron concentration at the surface of the extrinsic base region, inhibiting surface recombination, and improving the current gain of the device.

Fig. 1 is a schematic structural view of the present invention. The figure shows the edge of the emitter mesa, with emitter 1, base 2, N+ emitter 3, P-type base 4, N- collector 5, N+ substrate 6 and collector 7. There is a _SiO2 passivation layer 8 on the edge of the emitter mesa, and on the surface of the outer base region between the edge of the _SiO2 passivation layer 8 and the base 2, a metal layer 9 with a certain thickness and length is formed by sputtering, and the metal Layer 9 and the surface of P-type base region 4 form a Schottky contact structure.

FIG. 2 is a schematic diagram of the Schottky contact structure between the metal layer 9 and the base region 4 . This structure is only used to form a Schottky barrier, in which the metal layer is not used as an electrode, it is floated, and is not connected to any voltage and ground.

It should be emphasized that the P-type base region adopts a two-layer structure with different doping concentrations, and the base region with a thickness of t ₁ close to the emitter junction adopts low-concentration P-type doping, and the concentration is less than 10 ¹⁵ cm ^{- 3} ; another layer close to the drift region is the base region with a thickness of t ₂ , and its doping concentration is higher than that of the base region of the previous layer, reaching the order of 10 ¹⁷ cm ^-3 . A layer of light doping near the emitter junction is to adjust the work function of the P-type SiC semiconductor on the surface of the outer base region, so that the work function of the metal is greater than that of the semiconductor, so that the Schottky potential shown in Figure 4 can be formed Barrier, so at the same time can also improve the injection efficiency of the emitter junction.

Figure 3 is the energy band diagram before the Schottky contact, where _E0 is the vacuum energy level, (EF) _m is the Fermi level of the metal, _Wm is the work function of the metal, x is the electron affinity of the semiconductor, W _s is the work function of the semiconductor, (EF) _s is the Fermi level of the semiconductor, EC and EV are the conduction band and valence band of the semiconductor, respectively.

Figure 4 is the energy band diagram after the metal and P-type semiconductor are in contact. When the work function Wm of the metal is greater than the work function Ws of the P-type semiconductor, the energy band on the surface of the semiconductor is bent upwards to form a P-type anti-blocking layer (to prevent electrons from moving to At the same time, in order to ensure that the emitter junction is forward-biased, the base must be forward-biased, which will cause the energy band of the outer base region away from the surface to move down, and the energy band at the surface will further bend upwards, increasing the barrier height. The final result is that the surface of the P-type extrinsic base region is in an accumulation state, the hole concentration increases, the electron concentration decreases, and the electrons and holes are separated in space. Due to the existence of potential barriers, the movement of electrons to the surface is restricted, thereby preventing the electrons injected from the emitter region into the base region from being captured by surface traps in the outer base region, reducing the surface recombination current and increasing the current gain.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Within the spirit and principles of the present invention, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present invention.

Claims (10)

1. A silicon carbide bipolar junction transistor, characterized in that a Schottky contact structure is formed on the surface of the outer base region between the emitter mesa edge and the base ohmic contact of the bipolar junction transistor. 2 . The silicon carbide bipolar junction transistor according to claim 1 , wherein the Schottky contact structure comprises a base region and a metal layer on the base region. 3 . 3. The silicon carbide bipolar junction transistor according to claim 2, wherein the Schottky barrier formed by the Schottky contact structure is a P-type anti-barrier layer. 4. The silicon carbide bipolar junction transistor according to claim 2, wherein the base region has a two-layer structure with different doping concentrations. 5. silicon carbide bipolar junction transistor according to claim 4, is characterized in that, the two-layer structure of described base region is: one layer near emitter junction is the base region that thickness is t ₁ , and it adopts P-type doping; another layer close to the drift region is a base region with a thickness of t ₂ , and its doping concentration is higher than that of the base region close to the emitter junction, reaching the order of 10 ¹⁷ cm ^-3 . 6 . The silicon carbide bipolar junction transistor according to claim 2 , wherein the metal layer is not connected to voltage or grounded. 7. The silicon carbide bipolar junction transistor according to claim 2, wherein the work function of the metal layer is greater than the work function of the surface of the base region. 8. The silicon carbide bipolar junction transistor according to claim 2, wherein the metal layer is made of nickel or platinum. 9. The silicon carbide bipolar junction transistor according to claim 2, wherein a SiO ₂ passivation layer is formed between the left end of the metal layer and the edge of the emitter mesa. 10. The silicon carbide bipolar junction transistor according to claim 9, wherein the SiO ₂ passivation layer has a thickness of 40-60 nm.